One example of a drive unit for a hybrid vehicle in which a plurality of the speed change units are connected in tandem is disclosed in Japanese Patent Laid-Open No. 2003-127681. According to the system disclosed in Japanese Patent Laid-Open No. 2003-127681, an internal combustion engine is connected with a carrier of a planetary gear mechanism, and a first motor/generator is connected with a sun gear of the planetary gear mechanism. Also, a ring gear is connected with a member of an input side of a transmission. The transmission is a geared transmission such as an automatic transmission. A member of an output side of the transmission is connected with a propeller shaft, and a second motor/generator is connected with the propeller shaft. Thus, according to this drive unit for a hybrid vehicle, the planetary gear mechanism functions as a distribution mechanism distributing an engine power to the first motor/generator and to the transmission. That is, a revolution frequency of the ring gear, i.e., an input revolution of the transmission connected with the ring gear is varied continuously by changing a revolution frequency of the first motor/generator. Therefore, the planetary gear mechanism and the first motor/generator function as a continuously variable transmission. Consequently, a total speed change ratio of the drive unit for the hybrid vehicle is determined by the speed change ratio of the planetary gear mechanism functioning as a continuously variable transmission and the gear stage of the transmission arranged on the output side of the planetary gear mechanism.
According to the drive unit disclosed in Japanese Patent Laid-Open No. 2003-127681, since the engine revolution can be varied continuously by changing the revolution frequency of the first motor/generator, the engine revolution is controlled to a revolution frequency where the fuel economy is optimum utilizing this function. In this case, the engine revolution and a drive torque are varied continuously. On the other hand, if a shifting operation of the automatic transmission is carried out, the gear stage thereof is changed discontinuously (i.e., stepwise). For this reason, if the speed change operation is carried out only by the automatic transmission, changes in the engine revolution and the drive torque have to be too abrupt so that a passenger in the vehicle may feel uncomfortable feeling. In order to avoid or minimize such a problem, it is conceivable to vary the speed change ratio of the distribution mechanism (i.e., the planetary gear mechanism) in the direction opposite to the direction of the shifting operation of the automatic transmission when carrying out a shifting operation of the automatic transmission.
On the other hand, the continuously variable transmission composed mainly of the distribution mechanism or the planetary gear mechanism is adapted to carry out a speed change operation by operating the first motor/generator as a generator or an electric motor to change the revolution frequency thereof. Therefore, if an electric power generated by the first motor/generator cannot be accepted by an electric storage device or the like, or if the electric storage device cannot supply electric power to the first motor/generator sufficiently, a speed change operation of the continuously variable transmission cannot be carried out as desired.
Thus, a speed change operation of one of the speed change units such as the continuously variable transmission may be restricted for some reason. As a result, a control of coordinating speed change operations of the plurality of speed change units may be interrupted and this may cause shift shocks.